Iron Oxide Copper Gold Research Articles

Nanoinclusions in zoned magnetite from the Sossego IOCG deposit, Carajás, Brazil: Implication for mineral zoning and magnetite origin discrimination

Compositional zoning is common in magnetite from different geological environments, but its formation mechanism remains controversial. Here, we characterize micron- to nano-scale textural and chemical variations in zoned magnetite from the Sossego iron oxide-copper–gold deposit (Carajás, Brazil) using electron probe microanalyzer (EPMA) and transmission electron microscopy (TEM). Lack of porosity and of a reaction front at both the micron and nanometer scales indicates that compositional zoning in magnetite is a pristine texture formed during crystal growth, rather than a secondary texture due to dissolution and reprecipitation reaction. TEM energy dispersive X-ray spectrometry analyses and mapping identify four types of nanoinclusions in zoned magnetite: 1) Mg-Fe-Al silicates, most likely amphibole, 2) Fe-Ti oxides, mainly ilmenite, 3) pyroxene, 4) Si-rich magnetite. The formation of ilmenite nanoinclusions in Sossego magnetite is possibly due to oxy-exsolution of ulvöspinel from Ti-rich magnetite, whereas nanoinclusions of other minerals likely formed by local supersaturation in the fluid boundary layer, followed by magnetite crystal entrapment. Compositional zoning in magnetite likely formed by a self-organization process where fluid composition fluctuations are feedback responses for an evolving fluid system far from equilibrium, rather than cyclic variations in external factors such as temperature and oxygen fugacity. Saturation of silicate minerals results in relative depletion of Si, Ca, and Al in boundary layer fluids, and formation of inclusion-poor zone depleted in these elements. Nanoinclusions in magnetite highlight the importance of textural characterization when using in situ chemical composition to discriminate the origin of magnetite. In addition, the assemblages of nanoinclusions in magnetite can be used to complement the discrimination of magnetite origins.

Exploration vectoring from the white mica spectral footprint in the atypical auriferous Lavra Velha deposit, San Francisco Craton, Brazil

The Lavra Velha deposit is a poorly understood gold deposit hosted by the 2.1 Ga Ibitiara Granitoid, within the São Francisco Craton (Brazil). Although not consensually agreed, Lavra Velha has been inserted into the Iron Oxide-Copper-Gold (IOCG) class. The major alteration footprint of the deposit involves an upward mineralogical zoning from widespread chlorite-epidote-calcite (propylitic) to white mica-hematite-quartz association with an intervening magnetite-tourmaline-quartz-white mica realm. The ore occurs mostly as massive lenses composed of magnetite, hematite, pyrite, tennantite, bismuthinite, digenite and native gold. Gold is present as inclusion within hematite, pyrite, quartz and cassiterite, in association with white mica, but also with supergene jarosite-goethite. The proximal footwall and hanging walls of the mineralization are commonly anomalous in Cr-Ni-Co and, where weathered, a talc-nontronite association is documented. Such features are considered to represent ultramafic rocks closely associated with mineralization. Reflectance spectroscopy results show that white mica, hematite and Fe-tourmaline are closely and spatially associated with gold mineralization, thus representing a proximal alteration mineralogy. Notably, white mica becomes progressively more phengitic (>2210 nm) as higher Au grades are laterally approached, whereas it has muscovitic composition (<2198 nm) where gold grades are waning. This chemical gradient can be used as an exploratory guide to find zones of higher-grade mineralization. The crystallinity of white mica reveals a strong zonation from the top to the bottom of the deposit defined by the downward variation from muscovite to illite. The passage zone from muscovite to illite represents potential sites for gold deposition. In weathered profiles, the spectral mapping of talc-nontronite would be key in detecting gold-related ultramafic rocks. This work found that SWIR hyperspectral measurement of white mica features provides an efficient means for vectoring to gold mineralization, at least to district scale. The unusual mineralogical signature of Lavra Velha is not diagnostic of any specific style of known deposit and further in-depth studies are required to decipher what kind of deposit it belongs.

The Jaguar hydrothermal nickel sulfide deposit: Evidence for a nickel-rich member of IOCG-type deposits in the Carajás Mineral Province, Brazil

The Jaguar Ni sulfide deposit (mineral reserves of 58.6 Mt at 0.95 wt% Ni) occurs along a regional fault zone in the Carajás Mineral Province in the Amazonian Craton, Brazil. This study presents the first detailed description of the Jaguar deposit and provides a basic framework to constrain the origin of the Ni sulfide mineralization. Jaguar nickel sulfide orebodies are associated with ductile-brittle hydrothermal alteration zones developed over felsic subvolcanic or granitic country rocks. Prominent features of the Jaguar deposit include: i. its location along regional scale deep fault systems, ii. extensive zones of biotite-chlorite alteration overprinted by magnetite-apatite-quartz alteration zones, iii. sulfide mineralization occurring as steeply dipping orebodies closely associated with magnetite-apatite rich bodies, iv. sulfides that are disseminated or form the matrix of breccias consisting of pyrite and millerite, with minor pentlandite, chalcopyrite and pyrrhotite, v. pyrite, magnetite and apatite occur mostly as euhedral to subhedral crystals enclosed in irregular aggregates of fine-grained anhedral millerite, vi. sulfide mineralization with high Ni (Ni/S ~ 0.15–0.30) and Co (Co/S ~ 0.005–0.015) contents, high Ni/Cu ratios (~5–25) and low Zn and PGE contents, vii. metal association consisting of LREE, Fe, U, P, Pb, Bi, Te, Ni, and Co, and viii. mineralized zones with high F contents and a positive correlation of F with S, Ni and P2O5. The geological setting and characteristics of the Jaguar deposit are similar to the iron-oxide-copper-gold (IOCG) deposits in the Carajás Mineral Province, indicating that they all belong to a regional hydrothermal system. The existence of nickel-rich members of IOCG type mineralizations in the Carajás Mineral Province was previously suggested for other deposits (e.g., GT34, Castanha) and interpreted as deeper portions of the IOCG mineral system. The close association of nickel sulfide mineralization in the Jaguar deposit with magnetite-apatite-rich breccias may be interpreted as an iron-oxide-apatite (IOA) member within the regional hydrothermal system. In contrast to most hydrothermal nickel deposits in the world, the Jaguar deposit is neither hosted by mafic-ultramafic rocks nor associated with black shales. Therefore, the Jaguar deposit is interpreted as an uncommon hydrothermal Ni deposit associated with the IOCG mineral system in the Carajás Mineral Province. Our results provide new insights for worldwide exploration for Ni sulfide deposits in geological environments different from those traditionally considered prospective.

Nanomineralogy of hydrothermal magnetite from Acropolis, South Australia: Genetic implications for iron-oxide copper gold mineralization

Abstract Magnetite is the dominant Fe-oxide at the Acropolis IOCG prospect, Olympic Dam district, South Australia. Complementary microbeam techniques, including scanning transmission electron microscopy (STEM), are used to characterize titanomagnetite from veins in volcanic rocks and Ti-poor magnetite from a granite body with uplifted position in the volcanic sequence. A temperature of 670 ± 50 °C is estimated for Ti-poor magnetite using XMg-in-magnetite thermometry. Titanomagnetite, typified by Ti-rich trellis lamellae of ilmenite in magnetite, also displays sub-micrometer inclusions forming densely mottled and orbicular subtypes of titanomagnetite with increasing degree of overprinting. STEM analysis shows nanoparticles (NPs) of spinels and TiO2 polymorphs, anatase, and rutile. These vary as dense, finest-scale, monophase-NPs of spinel sensu stricto in Ti-poor magnetite; two-phase, ulvöspinelhercynite NPs in primary titanomagnetite; and coarser clusters of NPs (hercynite±gahnite+TiO2-polymorphs), in mottled and orbicular subtypes. Nano-thermobarometry using ilmenite-magnetite pairs gives temperatures in the range ~510–570 (±50) °C, with mineral-pair re-equilibration from primary to orbicular titanomagnetite constrained by changes in fO2 from ilmenite-stable to magnetite+hematite-stable conditions. Epitaxial relationships between spinel and Fe-Ti-oxides along trellis lamellae and among phases forming the NPs support exsolution from magnetitess, followed by replacement via mineral-buffered reactions. Lattice-scale intergrowths between ulvöspinel and ilmenite within NPs are interpreted as exsolution recording cooling under O2-conserving conditions, whereas the presence of both TiO2-polymorphs displaying variable order-disorder phenomena is evidence for subtly fO2-buffered reactions from anatase (reducing) to rutile (more oxidizing) stabilities. Transient formation of O-deficient phases is retained during replacement of ilmenite by anatase displaying crystallographic-shear planes. Development of dense inclusion mottling and orbicular textures are associated with NP coarsening and clustering during vein re-opening. Fluid-assisted replacement locally recycles trace elements, forming gahnite NPs or discrete Sc-Ti-phases. Hydrothermal titanomagnetite from Acropolis is comparable with magmatic magnetite in granites across the district and typifies early, alkali-calcic alteration. Open-fracture circulation, inhibiting additional supply of Si, Ca, K, and so on during magnetite precipitation, prohibits formation of silician magnetite hosting calc-silicate NPs, as known from IOCG systems characterized by rock-buffered alteration of host lithologies. Obliteration of trellis textures during subsequent overprinting could explain the scarcity of this type of hydrothermal magnetite in other IOCG systems.

Mineralogical and geochemical constraints on the origin of the variable REE enrichments in the Kangdian IOCG province, SW China

Iron oxide copper–gold (IOCG) deposits worldwide have been documented to be variably enriched in REEs in addition to Fe, Cu and Au metals, but the key factors controlling REE enrichments have long been poorly constrained. In this study, we conducted mineralogical and geochemical investigations on the timing and genesis of the REE enrichments in the Lala and Dahongshan deposits, the two largest IOCG deposits in the Kangdian metallogenic province, SW China. Both deposits have comparable nature of hosting rocks and a paragenetic sequence composed of early magnetite, accompanied with regional sodic alteration, and late Cu-sulfide mineralization, associated with K-Fe alteration, but exhibit different styles and grades of REE enrichments. The REE enrichments in Lala is close to industry-level, presenting as monazite with subordinate bastnaesite, allanite, and apatite of which apatite is mainly associated with the Fe mineralization stage, while others are co-existing with the Cu mineralization stage. In contrast, REE enrichments in Dahongshan is dominated by apatite that is mainly associated with magnetite of the Fe mineralization stage. Our new U-Pb dating of monazite from the Lala deposit yields a Neoproterozoic (~850 Ma) age. Such an age is slightly younger than previously obtained molybdenite Re-Os and allanite U-Pb ages of ~ 1.05 Ga, indicating that it should represent a fully or partially reset age that is related to regional Neoproterozoic (890–740 Ma) overprint event. On the other hand, previous zircon U-Pb dating show that the REE enrichments in the Dahongshan deposit was formed at ~ 1.66 Ga.Our in-situ Nd isotopic analyses show that the Lala monazite has εNd(t) values (calculated at 1080 Ma) ranging from −7.5 to −2.9, comparable to those of hosting rocks (7.2–0.3 at 1080 Ma) and partially overlapping contemporaneous felsic intrusions (−7.2 to 4.4), indicating hybrid sources of REEs from hosting rocks and/or coeval magmas. On the other hand, apatite grains in the Dahongshan deposit have εNd(t) values (calculated at 1660 Ma) ranging from −17.8 to −3.5, broadly similar to Archean besement rocks (−16.7 to −7.0 at 1660 Ma) in the Yangtze Block, but different from hosting rocks (2.9 to 4.6 at 1660 Ma) and contemporaneous mafic rocks (−0.2 to 4.0). In combination with textural observations (e.g., monazite-bearing apatite in Lala), this study further confirmed that different nature of Cu mineralizing fluids is the key factor controlling mobilization and leaching of REEs from the hosting rocks with similar lithologies. Available studies regarding fluid inclusion indicated that the fluids in both deposits are broadly similar, rich in Cl- and CO2 (e.g., HCO3− and CO32−). However, the presence of abundant fluorite in the Lala deposit (absent in the Dahongshan deposit) implies that the fluids in Lala should be distinctly rich in F. This conclusion is also supported by fact that the apatite in Lala has F and F/Cl values much higher than those of Dahongshan. Therefore, the relatively F-rich nature, which was generally considered to be more capable for breakdown of REE minerals, plays a key role for the extensive REE mobilization and mineralization in the Lala deposit. This study highlights that variable REE enrichments in IOCG deposits were likely mainly controlled by REE budgets of wall rocks and the nature of mineralizing fluids.

Monazite as an Exploration Tool for Iron Oxide-Copper-Gold Mineralisation in the Gawler Craton, South Australia

The chemistry of hydrothermal monazite from the Carrapateena and Prominent Hill iron oxide-copper-gold (IOCG) deposits in the IOCG-rich Gawler Craton, South Australia, is used here to define geochemical criteria for IOCG exploration in the Gawler Craton as follows: Monazite associated with IOCG mineralisation: La + Ce &gt; 63 wt% (where La &gt; 22.5 wt% and Ce &gt; 37 wt%), Y and/or Th &lt; 1 wt% and Nd &lt; 12.5 wt%; Intermediate composition monazite (between background and ore-related compositions): 45 wt% &lt; La + Ce &lt; 63 wt%, Y and/or Th &lt; 1 wt%. Intermediate monazite compositions preserving Nd &gt; 12.5 wt% are considered indicative of Carrapateena-style mineralisation; Background compositions: La + Ce &lt; 45 wt% or Y or Th &gt; 1 wt%. Mineralisation-related monazite compositions are recognised within monazite hosted within cover sequence materials that directly overly IOCG mineralisation at Carrapateena. Similar observations have been made at Prominent Hill. Recognition of these signatures within cover sequence materials demonstrates that the geochemical signatures can survive processes of weathering, erosion, transport and redeposition into younger cover sequence materials that overlie older, mineralised basement rocks. The monazite geochemical signatures therefore have the potential to be dispersed within the cover sequence, effectively increasing the geochemical footprint of mineralisation.

Hydrothermal alteration and Cu–Co mineralization at the peripheral zone (Target H) of the Kitumba iron–oxide copper–gold system, Mumbwa District, Zambia

AbstractTarget H is a new Cu prospect discovered in the Mumbwa district of central Zambia. Copper mineralization in the Mumbwa district is linked to the hydrothermal system associated with the granite‐quartz syenite intrusions of the Hook batholith. The extent of the hydrothermal system has not been documented, but various Cu prospects, including Target H, have been discovered at the margins of the granite‐quartz syenite bodies. The objective of this study is to investigate the geological characteristics of the Target H prospect using petrographic, geochemical, and sulfur isotope data in order to understand the features of mineralization at the margin of the hydrothermal system. The Cu‐ and Co‐rich orebodies are hosted in siltstone and breccia correlated to the Kundelungu Group of the Neoproterozoic Katanga Supergroup rocks. The siltstone consists of alternating beds of sandstone and mudstone that are composed of diagenetic quartz, albite, dolomite, calcite, and muscovite. Breccia occurs parallel to the bedding planes and in the folded and fractured siltstone units. Lithostratigraphy is characterized by three domains: breccia, transition, and host rock, which are defined depending on the proportion of siltstone and breccia. The diagenetic minerals have been modified by hydrothermal alteration, resulting in alteration products controlled by the primary mineralogy of the host rocks: K‐feldspar and muscovite replace albite and muscovite in siltstone, while siderite and hematite replace dolomite and calcite in breccia. Hydrothermal calcite is observed in the veinlets with or without siderite and hematite. Copper mineralization is associated with hydrothermal alteration and occurs as chalcopyrite, bornite, and chalcocite in breccia and stockwork veins. Diagrams correlating geochemistry to lithology show that the bulk of Cu mineralization is hosted in the breccia. An anomalous concentration of Co occurs with Cu in breccia, but the two metals do not show similar enrichment and depletion behaviors. The Cu‐rich orebody forms in hematite‐dominated breccia domain, whereas Co‐rich orebody forms in siderite‐dominated breccia units in the transition domain. In addition to Cu and Co, geochemical data show significant enrichment of Fe, P, Ba, Mo, Pb, W, U, V, and light rare earth elements in the breccia. Sulfur isotopic values of sulfide minerals show lighter δ34S values between 5 and 9‰ in the breccia and heavy δ34S signatures of approximately 17–28‰ in the siltstone. The δ34S values measured from sulfides in the breccia were comparable to the values obtained from the Kitumba deposit, which is located at the center of the hydrothermal system. We propose a magmatic‐hydrothermal origin for the Cu‐ and Co‐rich orebodies in the breccia at Target H. Breccia was the conduit for iron‐oxide‐rich metal‐bearing fluids that originated from the Kitumba deposit. Cooling and reaction of the fluids with dolomite‐rich siltstone triggered a redox reaction that deposited Cu sulfide minerals and an anomalous concentration of Co.

Distinguishing IOCG and IOA deposits via random forest algorithm based on magnetite composition

Iron oxide-copper-gold (IOCG) and iron oxide-apatite (IOA) are two significant mineral deposit types with similar tectonic settings and hydrothermal alteration characteristics. There are huge differences in the geological setting, alteration system, and ore-forming fluid composition among IOCG and IOA deposits, leading to controversial genesis. Distinguishing between these two deposit types is significant to reveal the origin of IOCG and IOA systems. In this study, random forest (RF) was employed to classify IOCG and IOA deposits based on the chemical composition of magnetite measured by the electron probe microanalyzer (EPMA) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The obtained results show that (1) a relatively high overall classification accuracy (0.76 for EPMA data and 0.91 for LA-ICP-MS) was obtained via the RF, indicating that the elemental composition of magnetite can effectively distinguish IOCG and IOA deposits; (2) the performance of the RF model based on LA-ICP-MS data is better than that of EMPA data, indicating that the application of more geochemical variables is helpful in distinguishing IOCG and IOA deposits; and (3) the elements V, Mg, and Mn in EPMA data, and Si, Mg, and V in LA-ICP-MS data are identified as the key elements for distinguishing IOCG and IOA deposits.

Comparison of petrological and geochemical characteristics of three different types of Eocene copper-gold mineralization in eastern Iran

The Maherabad, Qaleh Zari, and Hired deposits in eastern Iran are porphyry (Cu-Au)-, Iron oxide copper–gold (IOCG)-, and reduced intrusion-related gold system (RIRGS)-type mineralization, respectively. They are located within the Lut block magmatic assemblage, which is the main metallogenic province in eastern Iran. The mineral assemblages and paragenesis of the deposits indicate that highest log fO2 and log fS2 contents belong to the Qaleh Zari deposit. In contrast, the lowest values of oxygen and sulphur fugacity are related to the RIRGS system in the Hired deposit. The Maherabad mineralization has moderate log fO2 and log fS2 contents relative to the Qaleh Zari and Hired deposits.Geochemically, these three deposits are related to different types of granitoids, which occurred within the short time interval from 37 to 41 Ma. The Maherabad and Qaleh Zari intrusions are related to oxidized magnetite-series granitoids, whereas the mineralization at Hired deposit is related to the reduced ilmenite-series granitoids. Magnetite-series intrusive rocks at Hired are barren. Intrusive bodies in the three studied sites are metaluminous, high-K calc-alkaline to shoshonitic, and formed in a volcanic arc setting. However, there are distinct differences in the radiogenic isotopic composition, content of some elements and elemental ratios reflecting the effect of different sources in the formation of intrusions. The reduced granite of the Hired deposit has the lowest Eu/Eu* of <0.7, P + Sr < 1000 ppm, and Sr/Y < 20 associated with (La /Yb)N < 12. Crustal contamination played an important role in magma evolution (87Sr/86Sri = 0.70627 and −εNd(t) = -2.60). P + Sr and Sr/Y contents in the productive oxidant magnetite-series intrusive rocks, such as the Maherabad and Qaleh Zari deposits, are more than 1500 ppm and 35, respectively. On this basis, petrological characteristic, geochemical indicator elements and mineral assemblage and paragenesis can be used for discrimination of productive and barren intrusions in the three different deposit types studied in this research. This can be used as an exploration criteria for the hydrothermal deposits associated with different magmatic intrusions in some areas.

UNRAVELING MINERALIZATION AND MULTISTAGE HYDROTHERMAL OVERPRINTING HISTORIES BY INTEGRATED IN SITU U-Pb AND Sm-Nd ISOTOPES IN A PALEOPROTEROZOIC BRECCIA-HOSTED IOCG DEPOSIT, SW CHINA

Abstract Precambrian iron oxide copper-gold (IOCG) deposits are generally encountered with multistage hydrothermal overprints and hence have complex isotopic records. Precise dating of ore-forming and overprinting events and assessment of time-resolved metal sources are fundamental for understanding ore genesis. Here, we quantify the evolution history by integrating in situ U-Pb dating of texturally constrained allanite and Sm-Nd isotope data of ores and major rare earth element (REE) minerals in the breccia-hosted Lanniping Fe-Cu deposit in Kangdian region, southwestern China. The economically mineralized breccia in Lanniping Fe-Cu deposit is characterized by pervasive and texturally destructive replacement of polymictic clasts, including host metasedimentary packages, the intruded dolerite, and pre-ore halokinetic breccia. Ore minerals in cements are mainly composed of magnetite, chalcopyrite, bornite, and variable amounts of REE-rich minerals (e.g., apatite and allanite/epidote). Two types of allanite were identified in ores. Type I prismatic allanite texturally intergrown with magnetite has a SHRIMP U-Pb age of 1728 ± 20 Ma (1σ), which matches a zircon U-Pb age of 1713 ± 14 Ma (2σ) for the dolerite clasts and provides the direct age constraint on the Fe-Cu mineralization event. Type II anhedral allanite shows complex zoning and is spatially associated with, but texturally later than, magnetite, apatite, and chalcopyrite. This type of allanite yields significantly younger SHRIMP dates of 1015 ± 33 (1σ) and 800 ± 16 Ma (1σ) for cores and rims, respectively, which correspond to discrete regional magmatic events and hence record hydrothermal overprint/remobilization events of ore minerals in the deposit. Integrated Sm-Nd isotope compositions of type I allanite, apatite, and whole ores generally align along the reference Sm-Nd isochron of 1728 Ma, further confirming the primary ore formation at ~1.7 Ga. Corresponding εNd(1728 Ma) values ranging from –2.8 to 0.3 are significantly higher than those of the host metasedimentary rocks (–9.5 to –6.2) but comparable to those of contemporaneous igneous intrusions (–0.3 to 5.3) in the region, demonstrating that REE components of the primary ores were dominantly sourced from rocks of mantle-derived affinity. Both cores and rims of the younger type II allanite grains have Nd isotope compositions consistent with the unique time-evolved line of the ~1.7 Ga ores, implying that REEs incorporated into type II allanite were ultimately sourced from the primary ores in this deposit. The combined texture, chemical, U-Pb, and Sm-Nd isotope data thus demonstrate that REE remobilization was localized during post-ore hydrothermal overprint with negligible external inputs of REEs to the primary ores in the Lanniping deposit. In this contribution, we not only date primary ore formation but also recognize several younger allanite generations that record internal metal redistributions in response to post-ore tectonothermal events. Our study highlights the potential of ore-associated REE minerals such as allanite for resolving the age of multiple stages of hydrothermal events in complex ore deposits by ion probe, provided that careful examination of textural and paragenetic relationship of ores is conducted. Our finding of these younger allanite generations also exemplifies the significance of evaluation on time-resolved metal input for better characterizing the evolution history of the IOCG deposits.

Using cokriging to predict metal recovery accounting for non-additivity and preferential sampling designs

Predicted geometallurgical performance is progressively being incorporated into mineral resource modelling as a means of improving efficiency, decreasing operating costs and reducing risk in mining operations. Most geometallurgical studies are still statistics-based rather than geostatistics-based. Machine learning techniques are increasingly used to predict geometallurgical response variables, but most of these approaches do not directly consider spatial relationships and rely on the assumption that assay and mineralogy data implicitly account for spatial relationships.The work presented here shows that the prediction of metallurgical recovery, a non-additive variable, can be achieved by cokriging the masses of metal in the feed and in the concentrate, both of which are additive. The critical components of this approach are modelling the mean values and the spatial correlation structure of the predictands and the choice of the cokriging variant to be used. The proposal is demonstrated by applying it to the Prominent Hill Iron Oxide Copper-Gold (IOCG) deposit, for which the mass of metal in the feed is available from abundant assay analyses but the mass of metal in the concentrate is available from only a very limited number of laboratory-scale batch flotation tests for copper sulphide ores. By using a linear relationship between the mean masses of metal in the feed and in the concentrate, cokriging is shown to avoid the biases caused by partially heterotopic and preferential sampling designs and to overcome the consistency and accuracy problems encountered in using traditional simple or ordinary cokriging.

Apatite texture and trace element chemistry of carbonatite-related REE deposits in China: Implications for petrogenesis

Apatite is a ubiquitous mineral in carbonatites, and incorporates a variety of trace elements including rare earth elements (REEs). In this study, the textural and chemical variations of apatite were examined in order to trace the magmatic and hydrothermal petrogenesis of three carbonatite-related REE deposits: Shaxiongdong, Miaoya, and Bayan Obo. Various apatite textures were revealed by cathodoluminescence and back-scattered electron imaging. Magmatic apatite, which occurs predominantly in samples from Shaxiongdong, is euhedral, and commonly shows oscillatory or growth zonation with a yellow–green luminescent core and a violet luminescent rim. Euhedral to subhedral metasomatic apatite from Miaoya and Bayan Obo has a turbid texture, with the majority of grains associated with exsolved monazite. Hydrothermal apatite from Bayan Obo, typically occurring as aggregates in close association with fluorite and barite, is anhedral, with green or light violet luminescence.The different apatite textures are characterised by distinct trace element compositions. Magmatic apatite contains the highest concentrations of Mn (avg. 457 ppm) and Sr (avg. 18,285 ppm) and is characterised by a steeply inclined REE chondrite-normalised pattern. Metasomatic apatite, which has undergone in situ dissolution–reprecipitation, contains lower Mn (avg. 272 ppm) and Sr (avg. 9945 ppm) concentrations. It is characterised by highly variable REE trends with an La/SmN ratio varying from 0.13 to 5.61, and lower average La/YbN, La/SmN, and Sr/Y ratios (46, 2.2, and 18, respectively) than magmatic apatite. Hydrothermal apatite that was precipitated from a fluid is characterised by convex upward chondrite-normalised REE distributions with the lowest La/YbN, La/SmN, and Sr/Y ratios (13, 0.69, and 5.8, respectively). The average concentrations of Mn and Sr in this apatite are 270 and 6610 ppm, respectively. There are no Eu anomalies (Eu/Eu* = 0.97) in the chondrite-normalised REE plots for any of the analysed apatite samples. The combined textural and compositional variations of apatite in the three deposits reflect diverse magmatic and hydrothermal processes, including: 1) mineral fractionation contributing to core–rim zoning within the Shaxiongdong magmatic apatite; 2) dissolution–reprecipitation inducing monazite precipitation in Miaoya and Bayan Obo metasomatic apatite; and 3) coprecipitation with fluorite and barite from fluids generating the Bayan Obo hydrothermal apatite.A compilation of published apatite compositions from other rock types demonstrates that trace element compositions of apatite can be used to differentiate crystallisation environments and differentiate apatite from other rock types. Apatite from carbonatite has high Sr, REEs, La/YbN, Th/U, and Sr/Y, and no Eu anomaly, compared with apatite from igneous silicate rocks (except ultramafic rocks), and iron-oxide copper gold (IOCG) or iron-oxide apatite (IOA) deposits.

Mineral systems and their putative link with mantle plumes

Abstract In this contribution, I discuss the putative link of mantle plumes with selected categories of mineral systems. Continental rifting and break-ups can be induced by the upwelling of mantle plumes, also resulting in the generation of a wide range of mineral deposits. These include magma-associated ores, anorogenic igneous events responsible for iron oxide–copper–gold (IOCG) deposits, carbonatites and hydrothermal-induced mineralization, as well as hydrocarbons, salt domes, petroleum and gas, and several mineral systems in continental passive margins. Amongst the magma-associated mineral systems, the Ni–Cu–platinum group element (PGE), Fe–Ti–V and Cr deposits are the economically most important, such as those of the Bushveld Igneous Complex in South Africa. Anorogenic magmas are generally alkaline and associated with IOCG mineral systems, as exemplified by the giant Olympic Dam and similar deposits in South America. Carbonatites are considered as a distal effect of hotspot mantle plumes, as shown by Mount Weld in Australia, which may be related to the Bushveld Superplume. Plume-related thermal anomalies are the principal factor for the inception of hydrothermal circulation and the genesis of a wide range of hydrothermal mineral systems in rift-related tectonic settings. These include large-scale sedimentary-rock-hosted metalliferous ores, such as sedimentary exhalative (SEDEX) deposits. A modern example of is provided by the Red Sea brine pools. Some key examples are presented in this paper.

Ferro-tschermakite with polysomatic chain-width disorder identified in silician magnetite from Wirrda Well, South Australia: A HAADF STEM study

Abstract Silician magnetite within ~1.85 Ga lithologies hosting the ~1.6 Ga Wirrda Well iron oxide copper gold (IOCG) prospect, South Australia, was examined at the nanoscale. The magnetite is oscillatory-zoned with respect to the density and orientation of nanometer-scale inclusions, among which Si-Fe-nanorods and Al-rich amphibole (as much as hundreds of nanometers long and tens of nanometers wide) form swarms along &amp;lt;111&amp;gt; directions in magnetite. The amphibole is identified as ferro-tschermakite (Ftsk) with the crystal-chemical formula: A(K0.06Na0.01)0.07B(Ca1.65Na0.35)2C(Fe2.072+Al1.64Mg1.15Ti0.06Fe0.043+Mn0.04)5T(Si6.48Al1.52)8O22W(OH)2. This contains single and double rows of a triple-chain silicate attributed to clinojimthompsonite (Cjt) as coherently intergrown (010) zippers along the entire length of the grains. High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) imaging and simulation of Ftsk and Cjt on the [001] zone axis provide direct visualization of crystal structures. These are defined by the 7- and 10-atom octahedron strips (B+C sites) and flanked by double- and triple-pairs of Si atoms (T sites). Remarkably, the sites for light cations and/or vacancies are clearly imaged as single and double, darkest, diamond-shaped motifs separating the octahedron strips showing that A cavities known in amphibole are readily depicted in the wider-chain silicate. I-beam models show that nanoscale intergrowths among the two silicates are coherent along zigzag chains of cations at the edges of the octahedron strips, with single and double rows of the triple-chain silicate corresponding to 1 and 1.5 unit cells of Cjt (27 and 41 Å intervals along the b axis). This type of polysomatic chain-width disorder is widely reported in Mg-rich pyriboles but is shown here in an Al-Fe-rich amphibole. The lack of planar defects and/or reaction fronts at mutual contacts between three-chain zippers and host amphibole indicates primary co-crystallization growth, promoted by the formation of the Si-Fe-nanorods. Co-crystallizing plagioclase is also preserved in close vicinity to the amphibole hosted by magnetite (from a few nanometers to micrometers apart). In contrast, the replacement of amphibole by phyllosilicates is recognizable as irregular swells along the (010) zippers and results in extensive chloritization of the amphibole during an overprinting event. Pressures of ~11.5 kbar are estimated using Al-in-hornblende nano-geobarometry and calculated Al content in Ftsk (3.16 apfu). Assuming the amphibole-plagioclase association buffered by host magnetite fulfills the textural equilibration criteria required for application of this barometer, we interpret the Ftsk nanoinclusions in magnetite as preserved evidence for amphibolite facies metamorphism affecting host lithologies at Wirrda Well with subsequent retrograde alteration during the ~1.6 Ga IOCG mineralizing event. Magnetite records petrogenetic processes by accommodating variable ranges of nanomineral inclusions and preserving them over geological time scales. HAADF STEM imaging is ideally suited to the depiction of crystal-structural modularity and also provides insights into the evolution of geological terranes with protracted histories.

Relationship between Selected Major, Minor, and Trace Elements in Iron Oxide–Copper–Gold Deposits, an Example from the Unique Sin Quyen Deposit (Lào Cai Province, North Vietnam)

Abstract —We study the relations between several selected elements present in the Sin Quyen IOCG deposit, Lào Cai, North Vietnam, and interpret the obtained correlations, especially with a coefficient higher than 0.7. The correlations with high coefficients are mainly observed for the elements belonging to the chalcophile group (Cu, Ag, Au, Te, and Bi) and for the relation between uranium and Ag, Au, Cu, Pb, and Bi. Although the S-, Fe-, and REE-bearing minerals are predominant in the studied deposit, no strong correlation between them and the other elements was observed, even with Cu. The phenomena are primarily explained based on the geochemical properties of the mentioned elements and the characteristics of IOCG deposits.

Multiple sulfur isotopes in post-Archean deposits as a potential tracer for fluid mixing processes: An example from an iron oxide–copper–gold (IOCG) deposit in southern Peru

Sulfur has multiple stable isotopes (32S, 33S, 34S, and 36S), but little research has been conducted to investigate the information that low abundance sulfur isotope ratios (i.e., 33S/32S and 36S/32S) may carry on ore-forming processes in post-Archean deposits. In this study, the Cretaceous Mina Justa iron oxide–copper–gold deposit, was selected to examine the capability of multiple sulfur isotopes to trace ore-forming processes in post-Archean mineral deposits. In situ secondary ion mass spectrometry was applied to the pyrite of the magnetite–pyrite stage in the deposit to characterize the spatial variation of multiple sulfur isotopes at the microscale. This revealed a clear co-variation trend between δ34S (= 1000 × [(34S/32S)sample/(34S/32S)V-CDT − 1]) and Δ33S (= δ33S − 1000 × [(1 + δ34S/1000)0.515 – 1]). Simple modeling suggests that the triple sulfur isotope data recorded is best explained by mixing magmatic and externally derived (e.g., basinal brine) fluids. The contribution of external fluids is also supported by the trace element distribution patterns in pyrite, as revealed by laser ablation-inductively coupled plasma-mass spectrometry. This study is the first attempt to use multiple sulfur isotopes to trace ore-forming processes in post-Archean deposits and demonstrates that multiple sulfur isotopes are a faithful recorder of ore-forming processes at the microscale, especially when multiple fluids mix.

Trace Element Geochemistry of Pyrite from Bitumen-Bearing Stratabound Cu–(Ag) Deposits, Northern Chile

The Coastal Cordillera metallogenic province of northern Chile comprises several styles of mineralization, including stratabound Cu–(Ag), iron oxide–copper–gold (IOCG), iron oxide–apatite (IOA), an...

Cu-Au mineralization of the Sin Quyen deposit in north Vietnam: A product of Cenozoic left-lateral movement along the Red River shear zone

The Sin Quyen iron oxide–copper–gold (IOCG) deposit is located in the southwestern part of the Sin Quyen Formation, close to the Red River shear zone. The deposit is controlled by the Sin Quyen fault, parallel to the Red River fault system in North Vietnam. U-Pb dating of zircon and uraninite and 39Ar/40Ar dating of biotite and K-feldspar in the Sin Quyen deposit showed that the mineralization was emplaced in two main phases: the first phase formed magnetite, uraninite, and allanite of Precambrian age, between 520 and 744 Ma; the second most important phase introduced Cu-sulfides with Au, this event formed chalcopyrite, pyrrhotite, and pyrite in the temperature range 320 ± 40 °C between 88 and 22 Ma. In this study, a tectonic model was presented to explain when and how the Sin Quyen IOCG deposit and some other Cu-Au deposits along the Ailao Shan Red River Shear zone occurred in Cenozoic time.

Fluid evolution of a hematite-dominated, magmatic-hydrothermal Cu-Au deposit at Qibaoshan, Shandong Province, China

Qibaoshan is a magmatic-hydrothermal, polymetallic ore deposit located to the northwest of Wulian district, Shandong Province, China. The deposit hosts high grade copper and gold, which are the primary targets of currently active open-pit mining. However, the type, origin and mineralization mechanism(s) at Qibaoshan are still debated. Here, we studied the mineral assemblages, hydrothermal alteration, paragenesis of mineralization, and fluid evolution of the Qibaoshan deposit. Qibaoshan shares many similarities with some iron oxide-copper–gold (IOCG) deposits such as Mantoverde (Chile). The ore bodies of the Qibaoshan deposit are hosted in and structurally controlled by breccias, and are composed almost invariably of chlorite, coarse specular hematite, quartz and carbonates with subordinate pyrite and chalcopyrite. Passing inwards from the deep and peripheral parts towards the central ore zones, the intermediate- to felsic-host rocks mainly follow a sequence of hydrothermal alteration consisting of pervasive potassic and later acid alteration (chloritic and sericitic) events. The acid alteration is associated with the first pulse of copper deposition; which is followed by the deposition of specular hematite + euhedral quartz; then the deposition of (2nd) chalcopyrite + pyrite; and finally, late calcite + siderite precipitation in the vein centerlines. Each successive stage of this vein paragenesis shows clear evidence of precipitation into open space.The fluid evolution at Qibaoshan shows a transition from early, highly saline (47 wt% NaCleq), high temperature (396 °C), and low pressure fluids during the acid alteration; towards a moderate salinity (26 wt% NaCleq), intermediate temperature (272 °C) and pressure fluid during hematite growth; and then towards a low salinity (11 wt% NaCleq), low temperature (77 °C) fluid during the precipitation of euhedral quartz. The initial, high temperature stage showed features indicative of magmatic-hydrothermal origin. This first sequence included a metal-bearing (iron > copper) fluid pulse. Over the course of this sequence, the fluids showed gradual cooling coupled with a gradual decrease in salinity, probably indicating dilution as the early magmatic fluids mixed with externally derived meteoric water. After that, another metal-bearing (copper > iron) fluid pulse resulted in abundant (2nd) copper deposition in the veins. This vein sulfide stage was then followed by a trend towards lower salinity (14 wt% NaCleq), moderate temperature (225 °C) fluids that drove the precipitation of late carbonates. Overall, these results suggest that Qibaoshan is a magmatic-hydrothermal, IOCG-like deposit, with at least two magmatic fluid pulses, each followed by subsequent cooling and dilution by mixing with meteoric water that was tied strongly to the ore deposition.

Stability of aqueous Fe(III) chloride complexes and the solubility of hematite between 150 and 300 °C

Three sets of experiments were performed to test the stability and geologic importance of ferric (FeIII) chloride complexes in acidic, Cl-rich solutions at 150–300 °C, p = psat. Experiment Set A used the change in solubility of AgCl(s) in the presence of FeCl3 or FeCl2 to determine the stoichiometry of Fe(III) and Fe(II) chloride complexes at ΣCl = 0.1–3.0 molal. Results show that FeCl4− and FeCl2(aq) are the dominant Fe(III) and Fe(II) species, respectively, at T > 200 °C and ΣCl ≥ 1 m. Set B experiments used the solubility of elemental gold as a redox sensor to determine the aO2 of the FeCl4−/FeCl2(aq) boundary and log K values of +6.56 and +7.19 for the following reaction at 250 and 300 °C, respectively: FeCl2(aq) + ¼O2(g) + 2Cl− + H+ = FeCl4− + ½H2O(l). Ferric chloride complexes are stable under conditions of aCl−, aO2, and pH where gold is soluble as AuCl2−. Set C experiments measured the solubility of hematite in NaCl-HCl solutions at 200–300 °C by preparing a series of silica tubes with identical matrix chemistry (0.9 m NaCl + 0.1 m HCl) and increasing concentration of FeCl3. Hematite saturation was constrained by the tube with the lowest FeCl3 concentration that precipitated hematite at high temperature. Set C results were used to compute log K values of +3.6, +5.2, and + 7.4 for the following reaction at 200, 250 and 300 °C, respectively: 0.5Fe2O3(s) + 4Cl− + 3H+ = FeCl4− + 1.5H2O.Hematite solubility as FeCl4− is independent of redox state, and increases quickly with an increase in temperature, increase in Cl− concentration (power of 4), and decrease in pH (power of 3). Concentrations in excess of 100 mg/L ΣFe are attainable under geologically realistic conditions. Once formed, FeCl4− is an extremely effective oxidizing agent, capable of destabilizing any sulfide mineral and dissolving Fe, Au, and other metals (e.g., Cu, Pt, Pd) as chloride complexes. In deep hydrothermal systems, FeCl4− is a more viable oxidant than dissolved O2 gas, although its presence requires a source area with abundant hematite and a lack of reductants such as organic carbon, sulfides, or silicate minerals containing ferrous iron. Dissolved ferric chloride could be an important and previously overlooked reactant in the formation of certain types of hematite-rich hydrothermal mineral deposits, including iron oxide-copper–gold (IOCG) deposits.